Heat exchanger and heat exchange element therefor



May 20, 1969 E. H. PRIEST EIAL I 3,444,355.

HEAT EXCHANGER AND HEAT EXCHANGE ELEMENT THEREFOR Filed Jan. 23, 1967 y1969 E. H. PRIEST ET L HEAT EXCHANGER AND HEAT EXCHANGE ELEMENT THEREFORFiled Jan. 23, 1967 Sheet United States Patent 3,444,855 HEAT EXCHANGERAND HEAT EXCHANGE ELEMENT THEREFOR Ernest Horace Priest and GordonMichael Priest, both of Beverley House, London Road, Horsham, Sussex,England Filed Jan. 23, 1967, Ser. No. 611,056 Claims priority,application Great Britain, Jan. 31, 1966, 4,102/ 66 Int. Cl. F24h 3/00;F28d; F28f 1/14 US. Cl. 126-116 4 Claims ABSTRACT OF THE DISCLOSURE Thisinvention relates to heat exchangers, and is especially applicable toheat exchangers which are internally fired and of large size, whethervertical or horizontal, although the invention can be used for any sizeand many methods of heating or cooling. Again, this invention hasparticular, but not exclusive, relevance to heat exchangers for hightemperature work, i.e. with the heat exchanger arranged such that bothor all fluids passing through the heat exchanger (the fiuid(s) beingheated and the fiuid(s) being cooled) are at a temperature substantiallyabove 100 C., e.g. above 150 C., some time during their passage throughthe heat exchanger. Thus, for example, a heat transfer liquid such as adiphenyl/diphenyl oxide eutectic can be heated to temperaturesconsiderably higher than 150 C. and heat transfer liquid temperatures of400450 C., can be expected with special liquids. Gases could be heatedto as high as a 1000 C. or even 1500 C. if the heat exchanger issuitably constructed.

According to a first aspect of this invention, there is provided anextruded heat exchange element so formed that a plurality of suchelements can be joined together along their generallylongitudinally-extending sides to provide a first duct or group of ductssubstantially surrounding a longitudinally-extending second duct orgroup of ducts, the first duct or group of ducts being for passing afirst fluid through the heat exchanger in heat exchange relationshipwith a second fluid passed through the second duct or group of ducts.

According to a second aspect of this invention, there is provided a heatexchanger built from a plurality (e.g. at least three or four) ofextruded heat exchange elements, the elements, initially separate, beingjoined together along their generally longitudinally-extending sides toprovide a first duct or group of ducts substantially surrounding alongitudinally-extending second duct (e.g. a centrally-disposed duct) orgroup of ducts, the first duct or group of ducts being for passing afirst fluid through the heat ex- "Ice changer in heat exchangerelationship with a second fluid passed through the second duct or groupof ducts, with at least a substantial proportion of the heat transferredbetween the first and second fluids passing through the walls of saidelements.

Thus the overall dimensions of the ducts or group of ducts can bealtered by altering the number of heat exchange elements used, and thisenables different sizes of heat exchanger to be built using the samebasic elements. The individual elements can be arranged to be connectedtogether in many different ways, for instance by bolting along flangesprovided on the elements, or by clamping the elements with externalrings or by any other form of clamp, or by welding; if desired, theindividual elements need not be directly in contact with one another,but can be joined together by way of intermediate material, for instancesealing material and members for holding the sealing material inposition. Thus exact mating of the longitudinal sides of the elements isnot essential.

The cross-section of the elements can be as desired; if, as ispreferred, the elements are of hollow section closed on alllongitudinally-extending sides, the elements can be for instance squareor rectangular or trapezoid-shaped with the longitudinally-extending endsides converging (in transverse cross-section) such that they make thesame angle with the longitudinally-extending parallel sides with a viewto joining the elements together along their respective end sides, oreach element can be in the shape of an arc defined between twoconcentric circles and two radii with a view to joining the elementsalong their respective sides defined by the radii. An advantage of usinghollow section elements closed on all sides is that the extrudedelements need not have any welded seams and can have good resistance topressure, say up to 200 lbs./sq. in. (about 14 kg./cm. Nonetheless, theelements need not be closed on all sides, as for instance two oppositesides may be left open with the remaining two sides supported by a frameor one side could be left open with the element having a semi-circularcross-section.

Using suitably designed elements, it is possible to form a heatexchanger without any external bracing.

Local overheating is not a serious problem when heating water, for anysmall bubbles of steam so formed are quickly re-absorbed by the waterwhen they reach a cooler mass of water. However, with heat-sensitivefluids such as some phenol-based heat-transfer liquids, localoverheating can lead to irreversible changes such as coking, and must beavoided. Thus, especially if the first fluid is sensitive to localoverheating and is being heated by a hotter second fluid, the walls ofthe elements separating said first duct or group of ducts from thesecond duct or group of ducts may have their surfaces facing said firstduct or group of ducts substantially extended by projections, such asfins or protuberances, and if the projections are suitably designed,they can be extruded with the elements. In general, any heat-transfersurface of the heat exchanger can be provided with such projections.

As extrusion can be a relatively cheap way of fabricating an element,the exchanger can be of any suitable size, with the sizes of individualelements chosen as appropriate. Thus a small heat exchanger of 2 ft. 6ins. (about cms.) diameter could be built up of elements 4 ft. (aboutcms.) long of A inch (about 0.6 cm.) wall thickness, whilst a 10 ft.(about 3 metres) diameter heat exchanger could be built up of elements18 to 20 ft. (about 5.4 to 6 3 metres) long of inch to 1 /2 inch (about0. 6 cm. to 3.8 cms.) wall thickness, depending on the pressures to beresisted. With such a heat exchanger, it may eventually be possible toproduce capacities greater than million B.t.u. per hour, and possiblyeven up to 100 million B.t.u. per hour.

The elements are preferably joined together in an annular configuration,the term annular not referring solely to shapes defined betweenconcentric circles but also to shapes defined between polygons, thepolygons not necessarily being geometrically similar or concentric, andbetween non-concentric circles; however, for convenience, the exchangerwill normally have a concentric form, in transverse cross-section.

Further elements (preferably also extruded) may be built up around thefirst duct or group of ducts, to provide a third duct or group of ductsin order to reduce the height of a vertical exchanger or the length of ahorizontal exchanger, or even to put a third fluid in heat exchangerelationship with one of the other two fluids; a multipass exchanger isparticularly appropriate for heating gas, as there is little radianttransfer, the heat being mostly transferred by conduction andconvection.

The fluid need not flow in the same direction in adjacent ducts, astransfer pipes can be used to extend the flow path through the exchangerby providing a zig zag flow through a group of ducts, the ducts being inseries or parallel or series/ parallel. Thus with series connection, asmall quantity of the first fluid can be greatly increased intemperature by being passed through the heat exchanger under a highhead; with parallel connection, a large quantity of the first fluid canhave its temperature raised a small amount by being passed through theheat exchanger under a low head. The different ways of connecting thetransfer pipes gives great flexibility in the characteristics of theheat exchanger.

Tubes may be provided extending down the elements for passing the secondfluid, or even a third fluid, through the middle of the first fluid.

The invention will be further described, by way of example, withreference to the accompanying drawings in which:

FIGURE 1 is an axial section through a double-pass heat exchanger inaccordance with the invention, parts of the heat exchanger being omittedfor clarity;

FIGURE 2 is a cross-section along the line II-II of FIGURE 1; and

FIGURE 3 is a scrap cross-section through another heat exchanger inaccordance with the invention, on a larger scale.

The heat exchanger shown is fired by a burner producing hot gaseousproducts of combustion; such burners are well known in the art, and onlythe discharge orifice 1 is indicated in FIGURE 1. In FIGURE 1, the pathof the fluid being heated is indicated by single-headed arrows whilstthe path of the hot products of combustion is indicated by double-headedarrows.

The heat exchanger of FIGURES 1 and 2 is built up of two annuli ofextruded elements 2, 3 joined together along theirlongitudinally-extending sides. It will be noted that the elements 2 areof trapezoid section, and their longitudinally-extending sides areparallel, whereas the elements 3 are of rectangular section, leavingwedgeshaped gaps between their longitudinally-extending sides; thesewedge-shaped gaps can be filled with sealing material or refractorymaterial as desired.

One disadvantage of the trapezoid section elements 2 is that they onlyabut each other fully in one size of crosssection, i.e. in thetwelve-sided cross-section shown. For this reason, it may be moreconvenient to use rectangularsection elements throughout.

The outer annulus of elements 3 may, if desired, be enclosed by arefractory cover (not shown), but is shown 4 enclosed by an insulatingcover 4 (say of fibre glass) and a casing 4a.

Longitudinal fins 5 are extruded with the elements 2 and project intothe interior of the elements 2 from their radially inner walls. Fins 6are mounted on the end plate 7 to prevent any local overheating at thislocation. In general, fins may be provided on all the heat-exchangesurfaces in order to improve efliciency.

Circular-section tubes 8 extend down the interior of the elements 2 and3 of both inner and outer annuli, from the end plate 7, passing throughthe intermediate plate 9 and extending up to the other end plate 10.

A refractory cover 11 is provided adjacent the end plate 7.

As indicated by the arrows, the hot products of combustion issue fromthe discharge orifice 1, pass down the centrally-disposed duct in themiddle of the inner annulus of elements 2, and then return along the tworings of tubes 8 to issue from a stack 12. The fluid being heatedfirstly passes in counter-current to the hot products of combustion downthe outer annulus of elements 3, and then back along the inner annulusof elements 2.

The heat exchanger of FIGURES 1 and 2 is a doublepass heat exchanger; inanother possible embodiment of the invention, the heat exchanger may beformed as a single-pass heat exchanger, and having read the abovedescription, it would be apparent how to construct a singlepass heatexchanger in accordance with the teachings of the present invention.

Though a multi-pass heat exchanger is desirable when heating using gas,which has a low radiant output, a singlepass is possible when the heatexchanger is oil-fired, giving a high radiant output. Furthermore, itmay be possible to have only a single annulus of extruded heat exchangeelements in an oil-fired heat exchanger.

FIGURE 3 illustrates a modification of the heat exchanger of FIGURES 1and 2. The annulus 2 of FIG- URE 2 is replaced by an annulus 13 of heatexchange elements 14; the number of elements 14 depends on for instancedesired capacity and flow rates, but may be for example from 20 to 200or 300; the outside diameter of the tube of the elements 14 may be forexample from one inch to four inches or more. The annulus 3 of FIGURE 2is omitted, a containing wall 15 being mounted around the annulus 13.Otherwise, parts in FIGURE 3 corresponding to parts in FIGURES 1 and 2are indicated with the same reference numerals.

Each element 14 is extruded as a tube 16 having internal fins 17 andexternal longitudinal flanges 18. If desired, the flanges 18 may beWelded on after extruding the tube 16 and fins 17. Adjacent elements arewelded together at 19, along their flanges 18, to build up the annulus13. If desired, the gas tubes 8 within the elements 14 may contact thefins 17, for support, but this is not essential.

Thus though the invention has been particularly described above, it isonly to be limited according to the spirit and scope of the appendedclaims.

We claim:

1. An internally-fired heat exchanger comprising a casing, a burner forproviding hot combustion gases; a longitudinally extending centralduct'in communication with said burner; and a plurality of individualextruded heat exchange elements of hollow cross-section closed on alllongitudinally extending sides and joined together to form at least oneannular group of ducts within 'said casing for fluid to be heated; saidfluid being passed in heat exchange relationship with said hotcombustion gases with a substantial proportion of the heat transferredbetween said fluid and gases passing through the walls of said heat exchange elements separating the ducts of said group from saidlongitudinally extending central duct, said walls having integrallyextruded projections protruding into the ducts of said group to increasethe heat transfer surface area.

2. A heat exchanger as claimed in claim 1 wherein said projections arelongitudinally extending fins, and the crosssectional shape of the ductsof said group is trapezoidal.

3. A heat exchanger as claimed in claim 1 further comprising tubesextending through the approximate middle of at least one said elementsfor passing said hot combustion gases in heat exchange relationship.

4. A heat exchanger as claimed in claim 1 wherein said hot combustiongases provide radiant heat in said longitudinally extending centralduct, the surfaces of said walls defining said central duct being plain.

References Cited UNITED STATES PATENTS 8/ 1946 Villier 165-483 12/1953Stutz 165-154 1/1959 Soderstrom 165155 X 10/1964 Strache 165-155 6/1965Hahn 126116 ROBERT A. OLEARY, Primary Examiner. 10 T. W. STREULE,Assistant Examiner.

US. Cl. X.R.

